The invention relates to systems and methods for communications of a signal containing information, and more particularly to systems and methods for coding a signal containing, e.g., stereo audio information, to efficiently utilize limited transmission bandwidth.
Communications of stereo audio information play an important role in multimedia applications, and Internet applications such as a music-on-demand service, music preview for online compact disk (CD) purchases, etc. To efficiently utilize bandwidth to communicate audio information in general, a perceptual audio coding (PAC) technique has been developed. For details on the PAC technique, one may refer to U.S. Pat. No. 5,285,498 issued Feb. 8, 1994 to Johnston; and U.S. Pat. No. 5,040,217 issued Aug. 13, 1991 to Brandenburg et al., both of which are hereby incorporated by reference. In accordance with such a PAC technique, each of a succession of time domain blocks of an audio signal representing audio information is coded in the frequency domain. Specifically, the frequency domain representation of each block is divided into coder bands, each of which is individually coded, based on psycho-acoustic criteria, in such a way that the audio information is significantly compressed, thereby requiring a smaller number of bits to represent the audio information than would be the case if the audio information were represented in a more simplistic digital format, such as the PCM format.
In prior art, a stereo audio signal including a left channel signal (L) and a right channel signal (R) may be further encoded to realize additional savings in transmission bandwidth. For example, a stereo audio signal may be further encoded in accordance with a well known adaptive mean-side (M-S) formation scheme, where M=(L+R)/2 and S=(Lxe2x88x92R)/2. Such a prior art scheme takes advantage of the correlation between L and R, involves selectively turning on or off the M and S formation in each time domain block of the stereo audio signal for each coderband, and yet ensures meeting certain biaural masking constraints. It should be noted that in the adaptive M-S formation scheme, M provides a monophonic effect of the stereo signal while S adds thereto a stereo separation based on the difference between L and R. As such, the more separate L and R, the more bits are required to represent S. However, in a narrow band transmission, e.g., via a 28.8 kb/sec Internet connection, which is common, an M-S encoded stereo audio signal is undesirably susceptible to aliasing distortion attributed to the limited transmission bandwidth. Alternatively, by sacrificing the S information in favor of the M information in the narrow band transmission, mode distortion is introduced to the received signal, thereby significantly degrading its stereo quality.
Another prior art technique for further encoding a stereo audio signal to save transmission bandwidth is known as the intensity stereo coding. For details on such a coding technique, one may refer to: J. Herre et al., xe2x80x9cCombined Stereo Coding,xe2x80x9d 93rd Convention, Audio Engineering Society, Oct. 1-4, 1992. The intensity stereo coding was developed based on the recognition that the ability of a human auditory system to resolve the exact locations of audio sources of L and R decreases towards high frequencies. Typically, it is used to encode the intensity or magnitude of high frequency components of only one of L and R. However, the resulting encoded information facilitates recovery of the high frequency components of both L and R.
In accordance with the invention, the representation of a composite signal (e.g., a stereo audio signal) for transmission, which includes a first signal and a second signal (e.g., L and R), contains first information derived from at least the first signal, and second information concerning one or more coefficients resulting from parametric coding of the second signal. The first signal may be recovered based on the first information, and the second signal may be recovered based on the first information and the second information.
Advantageously, because of the coefficients used in the representation of the composite signal in accordance with the inventive parametric coding, the transmission bandwidth is efficiently utilized for communicating the composite signal. In addition, due to the design of the parametric coding, such coefficients describe not only an intensity relation between the first signal and the second signal, but also phase relations therebetween. As a result, the signal quality afforded by the inventive parametric coding is superior to that afforded, e.g., by the intensity stereo coding described above.